The Korean War was a conflict between north and South Korea. You could also say it was a conflict between the communist and the non-communist Korea. After the WWII Korea was separated into two parts by the 38th parallel. Soviet in north, which is the communist part, and the U.S in south, which is the non-communist part.
In 1948 rival governments were established. The republic part of Korea was now in south and the People's Democratic Republic of Korea in the North. The relationship between these two parts became worse, and on 25th. June 1950, North Korean forces invaded South Korea. The U.S condemned it was an act of aggression and told North Korea to take back their troops from South Korea, which they did not. So, on the 27th of June the president in the U.S, Harry Truman, allowed them to use sea and air forces in Korea to fight back.
Through the first weeks of the war North Korea didn't meet a lot of resistance and moved quickly forward. The North Korean troops had driven South Korea's army to Busan. Busan is at the southeast tip of Korea.
On September the 15th a counteroffensive began and The North Korean forces fell back. The supreme commander, MacArthur, was given the order to pursue them back again. They succeed, even though it only lasted for a few months. North Korea got help from Chinese communists and in January 1951 they recaptured the south Korean capital, Seoul.
The heavy fighting continued for months, but as the time went by the center of the conflict returned to the 38th parallel, where it stayed till the end of the war. At the end of the war the U.S had lost over 54,000 soilders and 103,000 were wounded. The Korean and Chinese losses were at least ten times as high.

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